There is a continuing need for improved devices and methods for determining one or more physiologic parameters of a patient. Often, such physiologic parameters are determined by detecting or measuring the quantity of concentration of an analyte associated with the physiologic parameter within a tissue of a patient. Noninvasive techniques for analyte detection are preferred over invasive detection techniques because invasive procedures result in stress and discomfort to patients. For example, conventional detection of blood analytes often involves drawing a sample of blood from the patient and subjecting the sample to ill vitro testing for a specific analyte. This technique suffers from a number of drawbacks. First, drawing blood requires the puncturing of a patient's skin and creates a risk of infection. Second, hypodermic needles used in drawing blood may also pose a risk of accidental infection to health care professionals such as phlebotomists who routinely use the needles and sanitary workers who handle contaminated needles. Third, the technique of drawing blood and in vitro testing is not easily adaptable for real-time and continuous monitoring of changes in analyte.
Nevertheless, analyte detection is of course an essential process in medical diagnosis. For example, it is necessary to determine the concentration of glucose in blood in diabetic patients on a regular basis. U.S. Pat. No. 5,771,891 to Gonzani describes a non-invasive blood analyte concentration monitoring apparatus for analyzing blood analytes such as glucose. This patent describes an apparatus that operates by stimulating an endogenous tissue with an electrical or magnetic stimulus, detecting a response of the tissue to the stimulus, correlating the response to a quantitative measure of the analyte concentration, and indicating the quantitative measure. It may also be desirable to measure the hematocrit concentration to determine the oxygen carrying capacity of a patient's blood. U.S. Pat. No. 5,372,136 to Steuer et al. describes a method for determining the concentration of a blood analyte such as hematocrit by passing at least two, preferably three, predetermined wavelengths of light onto or through body tissues such as the finger, earlobe or scalp and using mathematical manipulation of the detected values to compensate for spectral interference resulting from the presence of numerous species in body tissue.
There is also widespread interest in blood gas concentration for a number of reasons. For example, a patient may be rendered unconscious during surgery through administration of gaseous anesthesia such as nitrous oxide, desflurane, enflurane, halothane, isoflurane, methoxyflurane or sevoflurane. These gases are administered by allowing the patient to inhale the gases into his or her lungs. Once the gas is in the circulatory system, it passes through the blood-brain barrier into the brain where the gas increases the neurocellular threshold for firing. The partial pressure of gas in the blood provides an indication of the pharmacodynamics of the gas with respect to the brain such as change in neurological metabolic rate. Thus, analyzing blood with respect to anesthesia content is useful to ensure that the patient is neither overly sedated nor insufficiently anesthetized. Excessive sedation may cause permanent injury to the patient, sometimes resulting in death, and insufficient sedation is ineffective to suppress pain.
As another example, very low blood flow, or low “systemic perfusion,” occurs typically because of low aortic pressure and can be caused by a number of factors, including hemorrhage, sepsis, and cardiac arrest. The body responds to such stress by reducing blood flow to the gastrointestinal tract to spare blood for other, more critical organs. Thus, when blood flow from the heart is reduced, blood flow is generally maintained to critical organs, such as the brain, which will not survive long without a continuous supply of blood, while blood flow is restricted to less critical organs, whose survival is not as threatened by a temporary reduction in blood flow. For example, blood flow to the stomach, intestines, esophagus and oral/nasal cavity is drastically reduced when there is a reduced blood flow from the heart or when a patient is experiencing circular shock. For this reason, decreased blood flow to the splanchnic blood vessels provides an indication of perfusion failure in a patient. Physicians commonly take advantage of this phenomenon by taking CO2 and pH measurements in the stomach and intestine to assess perfusion failure.
Assessment of CO2 concentration in the less critical organs, i.e., those organs to which blood flow is reduced during perfusion failure, has also been useful in perfusion assessment. Carbon dioxide production, which is associated with metabolism, continues in tissues even during conditions of low blood flow. The concentration of CO2 builds up in tissues experiencing low blood flow because CO2 is not rapidly carried away. Correspondingly, O2 is consumed as CO2 is generated. This CO2 build-up (an increase in partial pressure of CO2 (pCO2)) in the less critical organs in turn results in a decrease in pH. Therefore, perfusion failure is commonly assessed by measuring pH or pCO2 at these sites, especially in the stomach and intestines. For examples of catheters used to assess pH or pCO2 in the stomach or intestines, see, e.g., U.S. Pat. Nos. 3,905,889; 4,016,863; 4,632,119; 4,643,192; 4,981,470; 5,105,812; 5,117,827; 5,174,290; 5,341,803; 5,411,022; 5,423,320; 5,456,251; and 5,788,631.
A number other patents discuss the measurement tissue analytes. U.S. Pat. No. 5,579,763 to Weil et al., for example, discusses a minimally invasive method for detecting a chemical characteristic in the gastrointestinal system of a patient who is in critical condition. In this patent, the specification focuses on taking measurements of carbon dioxide in a patient's esophagus using a catheter having a carbon dioxide sensor at its tip. Similarly, U.S. Pat. No. 6,055,447 to Weil also relate to carbon dioxide measurements and describes that the sensor may be placed against mucosal surface in the mouth or nose other than the sublingual area. In addition, U.S. Pat. No. 6,216,024 to Weil et al. discusses a device for assessing perfusion failure that comprises a carbon dioxide sensor for lying against a mucosal surface of the upper digestive/respiratory tract of a patient, an isolating means for inhibiting air flow around the mucosal surface, and indicating means operatively connected to the sensor for indicating a degree of perfusion failure of the patient.
Similarly, it is important to be able to accurately determine the amount of oxygen in the bloodstream or the amount of oxygen in the surrounding tissue. For example, pO2 in blood may be analyzed by using an oxygen electrode based on the so-called Clark's electrode system, described in U.S. Pat. No. 5,710,371 to Czernecki et al. Such electrode systems are based on electrochemical reduction of O2 at an anode made from an element such as tungsten or molybdenum. This type of analyte-sensitive portion requires the diffusion of O2 from blood across a membrane to a liquid electrolyte in which O2 is reduced at the anode/electrolyte interface, thereby generating an electrical current that corresponds to the pO2 in the blood sample. However, improper maintenance may subject the liquid electrolyte to evaporation or drying, thereby compromising the accuracy of analyte detection. Thus, this approach suffers from the drawback that the electrode apparatus must be meticulously maintained.
U.S. Pat. No. 5,423,320 to Salzman et al. describes a method for measuring or monitoring intraluminal gastrointestinal pCO2 and pO2. The method involves providing a catheter having a gas sensor, a CO2 and/or an O2 sensor, at a distal tip that is placed within a patient. The sensor is coupled to an output signal generator and/or recorder external to the patient. The catheter may, for example, be a nasojejunal or jejunostomy catheter, wherein the tip of the catheter is placed adjacent to patient tissue, e.g., stomach, colon, rectum, or jejunum tissue, to detect for tissue analyte in situ. Improper placement of such a sensor through use of the catheter may cause blanching of the tissue in which analyte is measured, i.e., occlusion of fluid flow therein, Moreover, during use, this type of device is highly susceptible to being inadvertently rotated to an improper angle or otherwise moved out of proper position for accurate analyte detection.
Thus, there is a need for a device to detect regional (local) or global (systemic) tissue perfusion in a number of contexts. Such devices may be used, for example, as a continuous monitor for extended time periods associated with surgical procedures and with recovery in an intensive care unit that involves a lengthy stay. Alternatively, a single-use version of such a device may be more appropriate for triage use in an emergency room or nursing home. In each of these applications, different CO2 and/or O2 sensing means and different product configurations may be more appropriate depending on the applications requirements for ease-of-use, cost, and response time. For example, either a single use sensor/probe that is discarded after every use or a reusable sensor/probe with or without a disposable sheath to minimize cross-contamination between patients may be utilized. In either case, an indicating means may be provided that is integral to the devices or linked to the device via an electronic, optical, or electromagnetic radiation means. However, since therapeutic decisions may be made based on the sensor readings, the sensor must accurately respond to analytes within tissue and not be compromised by ambient air or other contaminants.
There are a number of challenges associated with the accuracy of non-invasive detection of CO2 and/or O2 in exposed tissue. Referring to CO2 detection as an example, a patient's tissue and surrounding ambient air often exhibit large differences in their respective concentrations of CO2 and O2. If a tissue with a high concentration of CO2 is exposed to ambient air with a low concentration of CO2, a measured CO2 value at the surface of the tissue will have a concentration between the high and the low. This difference can be small or large depending on the how analyte from the deeper tissue diffuses to the surface tissue, how the analyte diffuses from the surrounding environment into the tissue, the physical properties of the tissue, and whether normal blood flow is maintained in the tissue.
It has now been discovered that various factors may compromise the usefulness or accuracy of measurement for certain tissue analytes associated with useful physiologic parameters, in particular, gaseous analytes such as O2 and CO2. For example, as noted above, tissue blanching tends to decrease the concentration of O2 in the blanched area, and a sensor may also be moved out of proper position during measurement. Thus, there is a need in the art for a device for accurate, useful yet noninvasive detection of an analyte, particularly a gaseous analyte, within a tissue of a patient. Such a device would be useful, e.g., to measure perfusion failure or to monitor the concentration of anesthesia during surgery.